Over the last few decades, the electronics industry has undergone a revolution by the use of semiconductor technology to fabricate small, highly integrated electronic devices. The most common semiconductor technology presently used is silicon-based. A large variety of semiconductor devices have been manufactured having various applicability and numerous disciplines. One such silicon-based semiconductor device is a metal-oxide-semiconductor (MOS) transistor.
The principal elements of a typical MOS semiconductor device are illustrated in FIG. 1. The device generally includes a gate electrode 101 (which is typically formed of polysilicon and will also be referred to herein as a polygate), which acts as a conductor, to which an input signal is typically applied via a gate terminal (not shown). Heavily doped source 103 and drain 105 regions are formed in a semiconductor substrate 107 and are respectively connected to source and drain terminals (not shown). A channel region 109 is formed in the semiconductor substrate 107 beneath the gate electrode 101 and separates the source 103 and drain 105 regions. The channel is typically lightly doped with a doping type opposite to that of the source 103 and drain 105 regions. The gate electrode 101 is physically separated from the semiconductor substrate 107 by an insulating layer 111, typically an oxide layer such as SiO.sub.2. The insulating layer 111 is provided to prevent current from flowing between the gate electrode 101 and the semiconductor source region 103, drain region 105 or channel region 109.
In operation, an output voltage is typically developed between the source and drain terminals. When an input voltage is applied to the gate electrode 101, a transverse electric field is set up in the channel region 109. By varying the transverse electric field, it is possible to modulate the conductance of the channel region 109 between the source region 103 and drain region 105. In this manner an electric field controls the current flow through the channel region 109. The current flow through the channel region is typically referred to as the source-drain current. This type of device is commonly referred to as a MOS field-effect-transistors (MOSFET).
Semiconductor devices, like the one described above, are used in large numbers to construct most modern electronic devices. A typical semiconductor device often includes a large number of transistors having different densities. For example, one region of the device may be more tightly packed than another and thus have higher density transistors. In order to increase the capability of such electronic devices, it is desirable to increase the speeds at which these devices operate.
One typical step for optimizing the speed of a semiconductor device is matching the drive currents of the various transistors. Semiconductor devices, however, typically include transistors of various density, some of which are so dense that the drive currents for the transistor cannot be measured directly. Accordingly, other techniques have been developed in an attempt to match drive currents.
One conventional technique which attempts to match the drive currents of transistors on a chip generally includes measuring the in-line widths of the polygates of the transistors on a test chip and determining the difference (often referred to as Poly .DELTA.W or the critical dimension) between these measured polygate widths and the polygate widths specified by the design specifications of the device. Under the assumption that the relationship between the drive current and the critical dimension is the same for transistors of different density, the process used to form the test chip is adjusted so that the critical dimensions of transistors on subsequently formed chips are equal. This is typically done by changing the mask used for polygate etching.